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Published online by Cambridge University Press: 27 October 2023
Let $\pi$ be a discrete group, and let $G$
be a compact-connected Lie group. Then, there is a map $\Theta \colon \mathrm {Hom}(\pi,G)_0\to \mathrm {map}_*(B\pi,BG)_0$
between the null components of the spaces of homomorphisms and based maps, which sends a homomorphism to the induced map between classifying spaces. Atiyah and Bott studied this map for $\pi$
a surface group, and showed that it is surjective in rational cohomology. In this paper, we prove that the map $\Theta$
is surjective in rational cohomology for $\pi =\mathbb {Z}^m$
and the classical group $G$
except for $SO(2n)$
, and that it is not surjective for $\pi =\mathbb {Z}^m$
with $m\ge 3$
and $G=SO(2n)$
with $n\ge 4$
. As an application, we consider the surjectivity of the map $\Theta$
in rational cohomology for $\pi$
a finitely generated nilpotent group. We also consider the dimension of the cokernel of the map $\Theta$
in rational homotopy groups for $\pi =\mathbb {Z}^m$
and the classical groups $G$
except for $SO(2n)$
.